Eren Yeager is fictional and the main character of the Attack on Titan series.
He’s a former member of the Survey Corps. Eren has only one goal- to end the war against Titans and kill them all. He wants to save humanity.
“If you win you live. If you lose you die. If you don’t fight, you can’t win.” – Eren Yeager
Human Form of Eren Yeager
Eren has green eyes, just like his father. He resembles his mother more than his father.
Up until the first 3 seasons, Eren is seen with short brown hair, his bangs parted at the center.
He has thick eyebrows and always has a determined look on his face.

When he was a child, pre-Scout Regiment era, he wore a green full sleeve shirt with black pants and a brown cardigan.
He always wore the key necklace that his father gave him.
When he finally became a soldier, he was almost always seen wearing the uniform with the Vertical Manoeuvring Equipment.
His uniform consisted of a short brown coat, white pants with the gear attached to it, and brown boots.
A green cape embellishes with the symbol of the Scout Regiment is worn on top of the attire. He still wears the key given by his late father.
He has a lean muscular body and is tall.
In the 4th season, Eren has longer hair with an indifferent look in his eyes. He has grown even more muscular and taller, his hair ties in a bun.
As an adult, he is seen wearing a beige suit with a white shirt underneath.
Titan Form

Eren is a tall Titan, seen without clothes and a muscular body.
He has elf-like ears, a long face with no flesh on his mouth which reveals his teeth and opens wide. His eyes are the same color as his human eyes.
He has long hair as a Titan. His Titan form is strong and heals very quickly.
Personality
Eren Yeager as a Human
Eren as a human is a determined, hot-headed, stubborn man who rushes into battle without thinking of the consequences.
Since childhood, he would always get into fights and Mikasa Ackerman would always be there to save him.
Since he was a child, he has been a fierce and strong-willed boy who had but one goal- to destroy every single Titan alive.
Eren is also incredibly smart, albeit air-headed at times and blinded by rage, he has the skills to analyze a situation and react accordingly.
It is also observed that he is emotional and very expressive of his emotions.
He is easily influenced by others’ emotions. He is also the kind of guy who would do anything to protect the people he loves and cares about.
He wants to protect humans at any cost. Since he was a child, he always wanted to be part of the Survey Corps, Scout Regiment.
He is also observant, and tactful. Throughout the series, Eren’s courage is always wavering- he is constantly reassured and uplifted by others around him.
He is almost always in danger, being the target of both humans and Titans, and is conflicted by his identity.
He is also curious- always questioning his father, and defying authority, believing that there is more to their world beyond the walls.
He is a man of principles and ideals.
Eren Yeager as a Titan
As a protagonist, he takes on the role of an anti-hero when he transforms into a Titan unexpectedly.
In his Titan form, he has a similar personality to his human form.
He’s always in fight mode when in Titan form and is enraged most of the time.
He over-exerts himself both as a Titan and as a human.
Eren’s Titan form, has the power of three Titans, the Attack and Founding Titan from his father and the War Hammer Titan from Willy Tyber.
There is not much difference between his titan personality and his human personality as his human form controls his titan form.
His Titan form is much stronger, he can lift boulders and has different powers which help him win fights.
History

Eren Yeager was born to Grisha Yeager and Carla Yeager in the Shiganshina District within Wall Maria.
He used to live with Mikasa Ackerman as his adopted sister. They are always seen together with Armin Arlert, their childhood friend.
Eren was close to his parents and Mikasa.
When Eren was very young, he saved Mikasa, who was kidnapped, and killed the kidnappers along with Mikasa.
Since then, Mikasa has been staying with the Yeager family.

Armin, Mikasa, and Eren have been close since childhood and as a child, he always dreamed of joining the Survey Corps.
They joined the military and Scout Regiments together. He joins the Scout Regiment early on, mainly because of his unexpected discovery as a Titan.
Levi Ackerman takes Eren under his wing. As the series Attack on Titan progresses, many events unfold that explains how and why he turned into a Titan.
Why did Eren turn evil?
Eren started to show his dark side and hatred for the titans significantly after the colossal Titan broke into Shiganshina, which gave paths for the titan to come inside the walls.
This is when they ate many people including Eren’s mother.
Even though Eren and Mikasa tried saving her, they begged them to let her go so that she would be able to protect them.

This can be said to be the main reason behind him turning to evil.
Similarly when he got to know about Carla’s death, the most pain Eren suffered in the manga series was here.
Seeing your friend die is surely the worst feeling.
Also it is seen that some of his friends he had for years in 104th cadet corps were killed when the titans invaded the Trost.
Also, he wanted to know what was in the basement of his childhood home.
But when he got to know about the same he was shocked and wasn't very happy with the answer.
He used to think that Titans were Eldian’s biggest enemy but this wasn’t true.
There were many people outside the Eldians who hated them and wanted to turn them to titans.
This led Eren to think about killing everyone outside of Paradis.